This invention relates to infrared radiation detection and more particularly to a solid state pyroelectric infrared radiation detection system.
An infrared image convertor or detector is a device for forming a visual image of a scene emitting infrared radiation. It may be likened unto an infrared analog of a television camera whch forms an image from visible light radiation. The uses of infrared detection systems and image converters are well known and include missile tracking, remote temperature measurement, viewing night scenes, etc.
Perhaps the most commonly used type of infrared detection systems at the present time are the so-called single element cryogenically cooled quantum detectors which utilize a mirror or system of mirrors to sequentially pan the points of a scene onto the detector. This type of detector, although very sensitive, is also very costly and difficult to maintain because of the need for cooling liquids.
Another type of infrared detection system utilizes pyroelectric material to detect incident infrared radiation, the signals of which may then be presented on some type of display device. In pyroelectric material, incident infrared radiation causes temperature changes and as a result a displacement current is generated which is proportional to the rate of temperature change. That is, infrared radiation incident on pyroelectric material causes the material to produce a changing polarization and surface charge thus giving rise to a displacement current. By appropriate measurement of the displacement current, an image can be produced of the incident infrared radiation. See for example "The Outlook for Pyroelectric Imaging Systems" by C. N. Helmick, Jr., Proceedings of the 1973 Electro-Optical Systems Design Conference, pages 195-207, and D. Pearsall, U.S. Pat. No. 3,581,092.
One requirement for pyroelectric infrared detection systems of the type mentioned above is that the pyroelectric material must be subjected to a temperature change or changes between readouts of the electrical charge patterns produced in the material since the displacement current in the material is produced by changes in temperature. That is, the infrared radiation falling on the pyroelectric material must in some manner be time modulated and this can be done by provision of an infrared optical chopper between the material and the scene producing the infrared radiation. The slower the chopping rate, the longer is the time the heat from the infrared radiation can integrate on the pyroelectric material to produce larger temperature changes and thus larger displacement currents. The result is better sensitivity, i.e., good ability to detect even low level infrared radiation. However, in previous pyroelectric detection systems, the slow chopping results in poor spatial resolution due to lateral thermal diffusion in the pyroelectric material. Thermal diffusion is simply the flow of heat from hot to cold areas giving rise to a smearing of the image. Slow chopping rates give the heat more time to flow from the hot to the cold areas before the signals are read from the pyroelectric material.
In solid state, pyroelectric infrared image detectors (those in which the pyroelectric material is mated with sold state readout electronics), slow chopping gives rise to another problem and this is that the heat produced in the pyroelectric material tends to flow through the mechanical electrode connections to the solid state electronics and thereby reduce the detected temperature changes in the material. The displacement current and the signals produced by the pyroelectric material is thus reduced. This problem was solved in low density pyroelectric arrays by mounting the pyroelectric material over an air gap and providing connection electrodes remote from the active detector area. This is feasible, however, only when the detector element spacing is 0.5mm or more. With higher density arrays, such as is possible with integrated circuit readout electronics, remote electrodes become impractical to use, especially with 2 dimensional arrays. Chopping the incident infrared radiation at a faster rate would also serve to obviate this problem since the heat would have less time, between sampling the signals from the pyroelectric material, to flow from the material into the solid state electronics. This would appear to be more economical and feasible than the previously mentioned spacing of connection electrodes; or of other techniques which have been tried such as making very low thermal conductivity electrodes.